測試檔案結尾
ReadFile函式會針對同步和非同步讀取作業，以不同方式檢查 E) OF (檔案結尾條件。 當同步讀取作業到達檔案結尾時， ReadFile 會傳回 TRUE ，並將 lpNumberOfBytesRead 參數指向的變數設定為零。 非同步讀取作業在起始對 ReadFile 的呼叫期間，或在後續非同步作業期間遇到檔案結尾，如果檔案指標是以程式設計方式進階超過檔案結尾。
下列 C++ 範例示範如何在同步讀取作業期間測試檔案結尾。
// Attempt a synchronous read operation.
bResult = ReadFile(hFile, &inBuffer, nBytesToRead, &nBytesRead, NULL);
// Check for eof.
if (bResult && nBytesRead == 0)
{
// at the end of the file
}
非同步讀取作業期間的檔案結束測試稍微比類似的同步讀取作業還要多。 非同步讀取作業的檔案結束指標是在 GetOverlappedResult 傳回 FALSE 且 GetLastError 傳回 ERROR_HANDLE_EOF時。
下列 C++ 範例示範如何在非同步讀取作業期間測試檔案結尾。
#include <windows.h>
#include <tchar.h>
#include <stdio.h>
#define BUF_SIZE (61)
LPCTSTR ErrorMessage( DWORD error )
// Routine Description:
// Retrieve the system error message for the last-error code
{
LPVOID lpMsgBuf;
FormatMessage(
FORMAT_MESSAGE_ALLOCATE_BUFFER |
FORMAT_MESSAGE_FROM_SYSTEM |
FORMAT_MESSAGE_IGNORE_INSERTS,
NULL,
error,
MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
(LPTSTR) &lpMsgBuf,
0, NULL );
return((LPCTSTR)lpMsgBuf);
}
void GoDoSomethingElse(void)
// Routine Description:
// Placeholder to demo when async I/O might want to do
// other processing.
{
printf("Inside GoDoSomethingElse()\n");
}
DWORD AsyncTestForEnd( HANDLE hEvent, HANDLE hFile )
// Routine Description:
// Demonstrate async ReadFile operations that can catch
// End-of-file conditions. Unless the operation completes
// synchronously or the file size happens to be an exact
// multiple of BUF_SIZE, this routine will eventually force
// an EOF condition on any file.
// Parameters:
// hEvent - pre-made manual-reset event.
//
// hFile - pre-opened file handle, overlapped.
//
// inBuffer - the buffer to read in the data to.
//
// nBytesToRead - how much to read (usually the buffer size).
// Return Value:
// Number of bytes read.
{
char inBuffer[BUF_SIZE];
DWORD nBytesToRead = BUF_SIZE;
DWORD dwBytesRead = 0;
DWORD dwFileSize = GetFileSize(hFile, NULL);
OVERLAPPED stOverlapped = {0};
DWORD dwError = 0;
LPCTSTR errMsg = NULL;
BOOL bResult = FALSE;
BOOL bContinue = TRUE;
// Set up overlapped structure event. Other members are already
// initialized to zero.
stOverlapped.hEvent = hEvent;
// This is an intentionally brute-force loop to force the EOF trigger.
// A properly designed loop for this simple file read would use the
// GetFileSize API to regulate execution. However, the purpose here
// is to demonstrate how to trigger the EOF error and handle it.
while(bContinue)
{
// Default to ending the loop.
bContinue = FALSE;
// Attempt an asynchronous read operation.
bResult = ReadFile(hFile,
inBuffer,
nBytesToRead,
&dwBytesRead,
&stOverlapped);
dwError = GetLastError();
// Check for a problem or pending operation.
if (!bResult)
{
switch (dwError)
{
case ERROR_HANDLE_EOF:
{
printf("\nReadFile returned FALSE and EOF condition, async EOF not triggered.\n");
break;
}
case ERROR_IO_PENDING:
{
BOOL bPending=TRUE;
// Loop until the I/O is complete, that is: the overlapped
// event is signaled.
while( bPending )
{
bPending = FALSE;
// Pending asynchronous I/O, do something else
// and re-check overlapped structure.
printf("\nReadFile operation is pending\n");
// Do something else then come back to check.
GoDoSomethingElse();
// Check the result of the asynchronous read
// without waiting (forth parameter FALSE).
bResult = GetOverlappedResult(hFile,
&stOverlapped,
&dwBytesRead,
FALSE) ;
if (!bResult)
{
switch (dwError = GetLastError())
{
case ERROR_HANDLE_EOF:
{
// Handle an end of file
printf("GetOverlappedResult found EOF\n");
break;
}
case ERROR_IO_INCOMPLETE:
{
// Operation is still pending, allow while loop
// to loop again after printing a little progress.
printf("GetOverlappedResult I/O Incomplete\n");
bPending = TRUE;
bContinue = TRUE;
break;
}
default:
{
// Decode any other errors codes.
errMsg = ErrorMessage(dwError);
_tprintf(TEXT("GetOverlappedResult failed (%d): %s\n"),
dwError, errMsg);
LocalFree((LPVOID)errMsg);
}
}
}
else
{
printf("ReadFile operation completed\n");
// Manual-reset event should be reset since it is now signaled.
ResetEvent(stOverlapped.hEvent);
}
}
break;
}
default:
{
// Decode any other errors codes.
errMsg = ErrorMessage(dwError);
printf("ReadFile GLE unhandled (%d): %s\n", dwError, errMsg);
LocalFree((LPVOID)errMsg);
break;
}
}
}
else
{
// EOF demo did not trigger for the given file.
// Note that system caching may cause this condition on most files
// after the first read. CreateFile can be called using the
// FILE_FLAG_NOBUFFERING parameter but it would require reads are
// always aligned to the volume's sector boundary. This is beyond
// the scope of this example. See comments in the main() function.
printf("ReadFile completed synchronously\n");
}
// The following operation assumes the file is not extremely large, otherwise
// logic would need to be included to adequately account for very large
// files and manipulate the OffsetHigh member of the OVERLAPPED structure.
stOverlapped.Offset += dwBytesRead;
if ( stOverlapped.Offset < dwFileSize )
bContinue = TRUE;
}
return stOverlapped.Offset;
}
int __cdecl _tmain(int argc, TCHAR *argv[])
// To force an EOF condition, execute this application specifying a
// zero-length file. This is because the offset (file pointer) must be
// at or beyond the end-of-file marker when ReadFile is called. For
// more information, see the comments for the AsyncTestForEnd routine.
{
HANDLE hEvent;
HANDLE hFile;
DWORD dwReturnValue;
printf("\n");
if( argc != 2 )
{
printf("ERROR:\tIncorrect number of arguments\n\n");
printf("%s <file_name>\n", argv[0]);
return;
}
hFile = CreateFile(argv[1], // file to open
GENERIC_READ, // open for reading
FILE_SHARE_READ, // share for reading
NULL, // default security
OPEN_EXISTING, // existing file only
FILE_FLAG_OVERLAPPED, // overlapped operation
NULL); // no attr. template
if (hFile == INVALID_HANDLE_VALUE)
{
DWORD dwError = GetLastError();
LPCTSTR errMsg = ErrorMessage(dwError);
printf("Could not open file (%d): %s\n", dwError, errMsg);
LocalFree((LPVOID)errMsg);
return;
}
hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
if (hEvent == NULL)
{
DWORD dwError = GetLastError();
LPCTSTR errMsg = ErrorMessage(dwError);
printf("Could not CreateEvent: %d %s\n", dwError, errMsg);
LocalFree((LPVOID)errMsg);
return;
}
dwReturnValue = AsyncTestForEnd(hEvent, hFile);
printf( "\nRead complete. Bytes read: %d\n", dwReturnValue);
CloseHandle(hFile);
CloseHandle(hEvent);
}
此範例程式碼的輸出如下所示。
ReadFile operation is pending
Inside GoDoSomethingElse()
GetOverlappedResult I/O Incomplete
ReadFile operation is pending
Inside GoDoSomethingElse()
ReadFile operation completed
Complete. Bytes read: 541